1. Field of the Invention
The present invention relates to an optical output module, an optical transceiver, and an optical transmission system.
2. Description of the Related Art
In recent years, in WDM (Wavelength Division Multiplexing) optical communication systems, the DQPSK modulation scheme has shown excellent characteristics such as high receiving sensitivity and long-distance transmission and there has been growing interest in increasing the performance of the optical communication systems.
FIG. 6 illustrates the basic configuration of the DQPSK optical transmitter 100. The DQPSK optical transmitter 100 includes a signal generating unit 102, a first amplifier 104, a second amplifier 106, a laser light source 108, an I phase modulator 110, a Q phase modulator 112, and a phase shift unit 114.
The first amplifier 104 generates an amplified signal of a first driving signal output by the signal generating unit 102 as a first modulation signal and the second amplifier 106 generates an amplified signal of a second driving signal output by the signal generating unit 102 as a second modulation signal. An optical signal output from the laser light source 108 is input to the I phase modulator 110, and the I phase modulator 110 modulates the phase of the optical signal based on the first modulation signal. In addition, the optical signal output from the laser light source 108 is also input to the Q phase modulator 112, and the Q phase modulator 112 modulates the phase of the optical signal based on the second modulation signal, in addition to which, the phase shift unit 114 performs phase shifting based on a bias voltage in order to set the phase difference of the two optical signals to 90°.
Next, the optical signal upon which phase modulation is performed by the I phase modulator 110 and the optical signal, upon which phase modulation is performed by the Q phase modulator 112 and phase shifting is performed by the phase shift unit 114, are combined and output as a DQPSK optical signal.
In such a DQPSK optical transmitter 100, since the optimum value of the bias voltage for the phase shift unit 114 to shift the phase of 90° drifts with respect to the ambient temperature and elapsed time, it is necessary to adjust the bias voltage to the optimum value.
Thus, in JP 2007-82094 A and JP 2010-204689 A, as shown in FIG. 7, a low-frequency pilot signal is given to the bias voltage. In addition, the photodiode 116 is set to receive the DQPSK optical signal. Further, a power synchronous detection unit 118 performs synchronous detection according to the frequency of the pilot signal so as to detect the power of the low-frequency component of the DQPSK optical signal. Thus, based on the power detected by the power synchronous detection unit 118, a bias control unit 120 adjusts the bias voltage so that the power of the low-frequency component of the DQPSK optical signal is minimized.
However, when adjusting the bias voltage as described above, it is necessary to add an RF (Radio Frequency) power detector to the power synchronous detection unit 118, or the like, and there are problems in that the scale of the hardware and the manufacturing cost are increased as a result.